Britain has told the Ecuadorean authorities it believes officials can enter its embassy in London and arrest Julian Assange, the founder of WikiLeaks, according to Ecuador's minister for foreign affairs, Ricardo Patino.

The development came two months after Assange walked into the embassy in a bid to avoid being extradited to Sweden where he faces allegations of sexual assault.

At a news conference on Wednesday, Patino said Ecuador would announce its decision regarding Assange's asylum request at 7am (noon GMT) on Thursday.

Patino also released details of a letter he said was delivered through a British embassy official in Quito, the capital of Ecuador.

The letter said: "You need to be aware that there is a legal base in the UK, the Diplomatic and Consular Premises Act 1987, that would allow us to take actions in order to arrest Mr Assange in the current premises of the Embassy."

The letter added: "We need to reiterate that we consider the continued use of the diplomatic premises in this way incompatible with the Vienna Convention and unsustainable and we have made clear the serious implications that this has for our diplomatic relations."

An Ecuadorean government spokesman said: "We are deeply shocked by the British government's threats against the sovereignty of the Ecuadorean embassy and their suggestion that they may forcibly enter the embassy.

"This a clear breach of international law and the protocols set out in the Vienna Convention.

"Throughout out the last 56 days Mr Julian Assange has been in the Embassy, the Ecuadorean government has acted honourably in all our attempts to seek a resolution to the situation.

"This stands in stark contrast to the escalation of the British government today with their threats to break down the door of the Ecuadorean embassy.

"Instead of threatening violence against the Ecuadorean embassy, the British government should use its energy to find a peaceful resolution to this situation which we are aiming to achieve."

A Foreign Office spokeswoman said: "We have consistently made our position clear in our discussions with the government of Ecuador.

"The UK has a legal obligation to extradite Mr Assange to Sweden to face questioning over allegations of sexual offences and we remain determined to fulfil this obligation.

"We have an obligation to extradite Mr Assange and it is only right that we give Ecuador the full picture.

"Throughout this process we have drawn the Ecuadoreans' attention to relevant provisions of our law, whether, for example, the extensive human rights safeguards in our extradition procedures, or the legal status of diplomatic premises in the UK.

"We are still committed to reaching a mutually acceptable solution."

Assange denies the allegations against him, but fears he will be sent to the United States if he goes to Sweden.

An offer to the Swedish authorities by Ecuador for investigators to interview Assange inside the London embassy was rejected.